The problem of controlling static charge during plastic web manufacturing and transport is well known. Generation and uncontrolled discharge of electrostatic charge can cause a number of serious problems including safety hazards. In the field of imaging, particularly photography and displays with an electrically modulated imaging layer, the accumulation of charge on surfaces leads to the attraction of dirt, which can produce physical defects. The discharge of accumulated charge during or after the application of the electrically modulated imaging layer or layer(s) can produce irregular switch patterns or “static marks” in the electrically modulated imaging layer. The static problems have been aggravated by increased sensitivity of new liquid crystals, increased coating machine speeds, and increased finishing operations. The charge generated during the coating process may accumulate during winding and unwinding operations, during transport through the coating or printing equipment and during finishing operations, such as slitting and spooling. Typical construction of displays with an electrically modulated imaging layer provide a clear flexible plastic web that is coated on one side with a highly conductive materials such as ITO and then coated with a liquid crystal layer that can change state upon the application of an electrical field. The flexible web used in most display applications may have a thickness of 3–7 mils and the side of the flexible web opposite the electrically modulated imaging layer does not have the capability to move or dissipate charge. When a web of this type is conveyed over rollers or through a roller nip, a residual charge is built up on the web. If the web is not grounded or sufficiently conductive, a charge will build up to the point where the surface can no longer hold the charge. If the electrical field is of sufficient level, it may cause the electrically modulated imaging layer to switch states (focal conic to planar or planar to focal conic) as a point source. The resulting web has a static induced mark that is in a different state than the surrounding background, creating a defect in the web. In many cases the switched portion of liquid crystal cannot be switched back easily.
It is generally known that electrostatic charge can be dissipated effectively by incorporating one or more electrically conductive “antistatic” layers into the support structure. The typical location of an antistatic layer is an external surface, which comes in contact with various transport rollers. For imaging elements, the antistatic layer is usually placed on the side of the support opposite the imaging layer. The imaging element may also have a dual system for control of static in which the resin-coated paper contains both ionically conductive salts and water, as well as a conductor on the outside of the backside resin layer.
A wide variety of electrically conductive materials can be incorporated into antistatic layers to produce a wide range of conductivities. These can be divided into two broad groups: (i) ionic conductors and (ii) electronic conductors. In ionic conductors, charge is transferred by the bulk diffusion of charged species through an electrolyte. Here, the electrical resistivity of the antistatic layer is dependent on temperature and humidity. Antistatic layers containing simple inorganic salts, alkali metal salts of surfactants, ionic conductive polymers, polymeric electrolytes containing alkali metal salts, and colloidal metal oxide sols (stabilized by metal salts), described previously in patent literature, fall in this category. However, many of the inorganic salts, polymeric electrolytes, and low molecular weight surfactants used are water-soluble and are leached out of the antistatic layers during processing, resulting in a loss of antistatic function. The conductivity of antistatic layers employing an electronic conductor depends on electronic mobility, rather than ionic mobility, and is independent of humidity. Antistatic layers which contain conjugated polymers, semiconductive metal halide salts, and semiconductive metal oxide particles, have been described previously. However, these antistatic layers typically contain a high volume percentage of electronically conducting materials, which are often expensive and impart unfavorable physical characteristics, such as color, increased brittleness, and poor adhesion to the antistatic layer.
A vast majority of the prior art involves the coating of antistatic layers from aqueous or organic solvent based coating compositions. For photographic paper, typically, antistatic layers based on ionic conductors are coated out of aqueous and/or organic solvent based formulations, which necessitates an effective elimination of the solvent. Under fast drying conditions, as dictated by efficiency, formation of such layers may pose some problems. An improper drying will invariably cause coating defects and inadequate adhesion and/or cohesion of the antistatic layer, generating waste or inferior performance. Poor adhesion or cohesion of the antistatic layer can lead to unacceptable dusting and track-off. A discontinuous antistatic layer, resulting from dusting, flaking, or other causes, may exhibit poor conductivity, and may not provide necessary static protection. Improper drying can also allow leaching of calcium stearate from the paper support into the processing tanks, causing build-up of stearate sludge. Flakes of the antistatic backing in the processing solution can form soft tar-like species, which, even in extremely small amounts, can re-deposit as smudges on drier rollers, eventually transferring to image areas of the photographic paper, creating unacceptable defects. Moreover, the majority of conductive materials used as antistats on current photographic paper products lose their electrical conductivity after photographic processing due to their ionic nature. This can cause print sticking after drying in the photo processor, and/or in a stack. Other imaging elements that are on resin coated paper bases have a dual system for control of static in which the paper contains both ionically conductive salts and water as well as a conductor on the outside of the backside resin layer.
In U.S. Pat. Nos. 6,197,486 and 6,207,361, antistatic layers have been disclosed, which can be formed through the (co)-extrusion method, thus eliminating the need to coat the support in a separate step and rendering the manufacturing process less costly.
With the development of all plastic web media, such as, for example, foam-core polymer sheets, the conductivity requirements of the plastic web media are typically increased because there is no paper base that contains water and salt to provide conductivity. The web media may require the addition of an electronically conducting material such as tin oxide, polythiophene and others. Some of these materials have color associated with them at various coverages. These materials are also very expensive when compared to more conventional conductive compounds.